The present invention is related to a process for heat treating a biaxially oriented film of a polyolefin, thermoplastic, crystallizable polymeric material and the resultant film.
A distinguishing characteristic of oriented packaging film is its capacity, upon exposure to some level of heat, to shrink or, if restrained, to create shrink tension within the film. The packaging film also has good transparency and gloss. In addition, it has excellent heat seal properties at low temperatures, sliding properties over a wide range of temperatures and releasing properties from a hot plate, all of which are desirable for packaging articles by the use of an automatic packaging machine.
In recent years, highly advanced requirements have been made in overwrapping and outerwrapping of a variety of products such as foods, tobacco, industrial goods and daily miscellaneous goods. In every instance, the appearance of the product is critical, especially on the shelf of a retailer. A critical requirement of wrapping film is that it has a low degree of shrinkage in order not to create variations in the film dimensions on aging. Variations in film dimensions over time can greatly detract from the appearance of packages wrapped in such materials either through distortion of the underlying package or by development of unsightly wrinkles in the film itself. Hence, dimensional stability of a wrapping film is critical to the appearance of the product that is wrapped.
There are many approaches in the prior art for achieving dimensional stability of oriented films. Commercial tenters are typically designed with a "heatset" section for achieving dimensional stability. This approach essentially involves decreasing the distance between the two tracks near the tenter's exit. By so doing, the film is permitted to contract at an elevated temperature so as to permit relaxation of residual stresses caused by orientation. However, film heatset in this way is prevented from undergoing any contraction in the longitudinal direction once its edges have been gripped by the tenter clips.
Other approaches for achieving dimensional stability of oriented films that have been proposed are, for example, a solvent absorption: desorption technique to reduce shrinkage at elevated temperatures in polyethylene terephthalate. By orienting at elevated temperatures, allowing unconstrained relaxation at low temperature, and finally constrained elevated temperature annealing, low shrink films made from polyethylene terephthalate have been prepared (Japanese patent 63,130,650). In addition, by orienting at temperatures above the polymer crystallization temperature, low shrink films made from polypropylene and ethylene-propylene copolymers have been manufactured (Japanese Patent 7,153,931).